1. Field of the Invention
The invention relates to a photo sensing device, more particularly, to a photo sensing device suitable for an optical touch display panel.
2. Description of Related Art
A man-machine interface (MMI) refers to a medium between a user and a machine used for communication. Electronic devices sold in the market are all designed with an MMI to facilitate user's operation, so that the user can execute various functions of the electronic devices. With development of information technology and quick development and applications of wireless mobile communication and information appliances, to achieve advantages of easy carry, small size and more user-friendly in operation, input devices of many electronic devices have been changed from keyboards and mice to touch panels, and a combination of a display panel and the touch panel makes a design of the MMI more convenient.
Presently, methods of combining the touch panel and the display panel are roughly grouped into a plug-in type and a built-in type, and when the touch panel and the display panel are combined in the built-in type, it avails reducing a volume of the electronic device to reach a volume thinning demand. Therefore, the technique of building the touch panel in the display panel has gradually drawn attention, and a technique of directly building an optical touch panel in the display panel to form an optical touch display panel have become mature and stable.
Generally, the optical touch panel built in the display panel has a plurality of photo sensing devices arranged in an array for sensing touch events. If a certain photo sensing device senses a touch event, the photo sensing device outputs a corresponding judging signal to a back-end processing circuit for processing/judging, so that the electronic device having the optical touch display panel executes a corresponding function.
Taking a photo sensing device having a characteristic of a light-shading sensing mode as an example, a touch operation of a finger may shade light above the photo sensing device. Therefore, the photo sensing device would output a corresponding judging signal to achieve a touch control function. To be clear, the photo sensing device having the characteristic of the light-shading sensing mode performs a touch sensing operation according to a light-shading status of the external light. Certainly, in case of a strong external light, since the external light sensed by each of the photo sensing devices is strong, a difference (i.e. a signal strength difference) between a judging signal associated with occurrence of the touch event and a judging signal associated with non-occurrence of the touch event that are output by each of the photo sensing devices is relatively great. In this way, the back-end processing circuit can accurately determine whether the finger performs a touch operation on the optical touch display panel according to an output of each of the photo sensing devices.
Unfortunately, in case of a weak external light, since the external light sensed by each of the photo sensing devices is weak, the difference (i.e. the signal strength difference) between the judging signal associated with occurrence of the touch event and the judging signal associated with non-occurrence of the touch event that are output by each of the photo sensing devices is relatively small. In this way, the back-end processing circuit may not accurately determine whether the finger performs a touch operation on the optical touch display panel. Therefore, the existing optical touch display panel cannot be widely used in conditions of different environmental lights.